Accurate selective detection and quantification of chemical species has to date not been achievable at a relatively low cost. Paper tape sensors coated with a compound which changes color when exposed to a chemical species can be relatively inexpensive. However, the tape coating layer may respond to a number of species, making selectivity difficult. In addition, the color change may be indicated at an extremely low concentration for one species, and a relatively high concentration for other species. In that case, the presence of a trace amount of the first species could be detected as a significant quantity of the second species. Since many species are present in the atmosphere in trace amounts, the potential for interference or false readings with these paper tape devices is thus significant.
Another category of relatively inexpensive chemical sensors are solid state devices, such as metal oxide films, which display a conductivity change in the presence of certain chemical species, but only at elevated temperatures. The solid state sensors are often used to detect toxic gases. Although adequate for the gross detection of certain species, these metal oxide sensors are typically responsive to many gases, leading to problems with accuracy and selectivity. In addition, because the devices operate at a high temperature, they require a power source for heating and can degrade quickly.
The sensing techniques which have the required sensitivity and selectivity are typically extremely sophisticated, expensive scientific instruments which need to be operated by a skilled technician. For example, mass spectrometers are very sensitive and can achieve the desired selectivity, unless two or more species present in the sample give rise to ions with the same mass to charge ratio. Spectroscopic techniques in principle can achieve the desired selectivity and sensitivity as long as there is no overlap in the species' spectroscopic features, but the instruments typically cost tens of thousands of dollars.